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Gravitational Lenses - Nature's Telescope

Scientists have proposed the existence of "dark matter" halos around
individual galaxies and clusters of galaxies. Dark matter is material
which affects the objects around it through gravity, but which emits
no light of any wavelength that we can detect. Astronomers suggest
that we see only the "tip of the iceberg" when we use our large
telescopes to look out into space.

A relatively new method used by astronomers is to look for a change in
the way light behaves, due to gravitational lensing. It is somewhat
akin to how you avoid running through sliding glass doors. If the
glass is very clean and you get no reflections, how can you tell the
door is there? Your eyes perceive that the light from outside is
bent, and infer that something is bending it, namely the door. Note
that you didn't see light FROM the door, but you saw the effect made
by the door on light from objects behind it. Similarly, we can use
telescopes to look at light from very distant objects behind the
clusters of galaxies we are interested in to study the clusters.

The possibility that the path of light could be bent by the gravity of
a large object was predicted by Einstein'sTheory of General
Relativity, and this effect was observed soon after the theory was
published. Because people normally think of glass or plastic lenses as
bending light, we call any massive object that bends light rays a
"gravitational lens." Astronomers now use specialized electronic
cameras on large telescopes to very carefully measure how much the
light from background objects is bent. By analyzing the amount of
bending, we can then determine the mass of whatever is doing the
bending.

In the cluster Abell 2218, distant blue galaxies behind the large
cluster of galaxies are "squished" into a circular shape around the
middle of the foreground cluster. By measuring the amount of
distortion in the more distant blue galaxies, we can determine that
there is indeed "dark" matter in the cluster! In fact, we can even
measure how much mass there is that we can't see -- this galaxy
cluster happens to have nearly 400 trillion times the sun's mass in
"dark" matter.

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